This invention relates to certain polyhydroxy acid compositions and films produced from these compositions, which are useful as degradable packaging materials.
High molecular weight polymers of hydroxy acids and cyclic dimer products of esterification of alpha hydroxy acids, particularly glycolic acid and lactic acid, are well known to be degradable, and are found useful for disposable packaging that will substantially and readily deteriorate to harmless byproducts under conditions existing in the natural environment or employed in suitable waste treatment facilities. It is desirable for packaging material to possess additional beneficial physical properties including optical clarity, tensile strength, resistance to puncture and, optionally, to be shrinkable. Shrinkable films are useful for "shrink-wrap" packaging. Shrink-wrapping is the process by which a film structure placed around an object contracts upon application at convective of radiant heat, to provide a tight, protective, self-sealing container. In order to make polyhydroxy acid films shrinkable, the polyhydroxy acid films must be oriented by mechanical stretching procedures.
Many high molecular weight polyhydroxy acids of commercial significance contain large proportions of specific asymmetrical, carbon atoms and may be, therefore, appreciably crystalline and brittle. Such polymers are less able to withstand the demanding film-forming and orientation processes without excessive film breakage causing low yield. Further, polyhydroxy acid films possessing an appreciable crystallinity can become brittle on aging.
A method of making a polyhydroxy acid film pliable is to plasticize the film by incorporating large amounts of monomeric and low molecular weight oligomeric alpha hydroxy acid species. However, such plasticized polyhydroxy acid compositions can be difficult to extrude into films because they stick to the film-making equipment, produce films of non-uniform thickness and the plasticizer often fouls the equipment. When successfully processed to film form, plasticized polyhydroxy acid compositions, by themselves, have glass transition temperatures, Tg, which are near ambient temperatures and may not be stable under storage conditions found in warm climates. In addition, plasticized polyhydroxy acid composition may not have an appropriate rate of degradation to maintain structural integrity for the intended useful life of the package, and, when subsequently oriented to shrinkable film, may not exhibit the heat set shrink force needed for successful application to the product or for good package strength.
Schneider, discloses in U.S. Pat. No. 2,703,316 the manufacture of films from lactic acid. Schneider's polylactic acid from commercial lactic acid was inherently racemic and therefore the film produced therefrom would suffer from defects similar to those of highly plasticized polyhydroxy acid compositions.
It is desirable to provide degradable compositions which may be processed economically, and with a high yield, into an optically clear, robust and, biaxially oriented film for commercially useful, disposable packaging applications.